For millions of years, the river Ebro has sloshed south from Spain’s jagged Cantabrian Mountains, carving out a broad valley that is now home to one of the country’s most fertile wine regions. Between its sprawling vineyards, the landscape rises steeply to hilltop medieval towns. Laguardia is the best known, on account of its high walls, cobblestones, and cavernous wine cellars. But the town’s rustic grandeur conceals a deep history of violence. More than 2,000 years ago, Celtic tribes fought a decades-long series of wars in this region, part of a brutal last stand against the invading Romans—and for Laguardia, even those conflicts were of relatively recent vintage.

Some years ago, just outside the town walls, workmen at a construction site were operating a bulldozer when one of them spotted bones sticking up through the disturbed earth. Archaeologists were dispatched to the scene. Careful brushwork revealed not one human skeleton but 90, along with pieces of more than 200 others, all dated to a little more than 5,000 years ago. A new analysis of the site led by the archaeologist Teresa Fernández-Crespo places these bones among the most spectacular finds in the anthropology of human warfare—but far from the oldest.

For nearly a century, anthropologists have wanted to know how long people have been engaged in organized group violence. It’s not some idle antiquarian inquiry. For many, the question bears on human nature itself, and with ruinous wars ongoing in Europe, the Middle East, and elsewhere, it has become more resonant. If warring among humans began only recently, then we might be able to blame it on changeable circumstances, cultural or otherwise. If, however, some amount of war has been with us since our species’s origins, or earlier in our evolutionary history, it may be difficult to excise it from the human condition.

Because war is, by definition, organized violence, some early anthropologists believed that it was invented by the first large-scale, sedentary societies. They were, after all, much more organized than their predecessors, and we know that many were warlike. Hieroglyphic inscriptions tell us that more than 5,000 years ago, the first pharaoh conquered chiefdoms up and down the Nile delta to consolidate his power over Egypt. A Sumerian poem suggests that some centuries later, King Gilgamesh fended off a siege at Uruk, the world’s first city. But new findings, at Laguardia and other sites across the planet, now indicate that wars were also occurring at small-scale farming settlements all the way back to the dawn of agriculture, if not before.

We often lament the fog of war for the way it obscures and distorts our view of distant conflicts. When archaeologists try to peer into prehistory, this fog thickens and becomes nearly impenetrable. Earth’s geological processes fossilize only one bone in every billion, according to one estimate. Locating evidence of violence on those that do survive intact can be a challenge. An arrowhead lodged into a rib is unambiguous, but such finds are rare. Bioarchaeologists have to look for cranial fractures or “parrying injuries” on forearms that a person raised in self-defense during their last moments. In recent decades, they have learned to distinguish trauma inflicted by the swing of a heavy club from the more splintery cracks sustained by bones that time has made brittle.

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Even in cases where scientists find a one-off skeleton from a person who clearly met a violent end, they can’t always be sure that the fatal blow was struck by a warrior from a neighboring tribe. It might have come from a romantic rival or jealous brother. To know that you’re looking at evidence of a war, it helps to have lots of skeletons. The site outside Laguardia had stacks upon stacks of them, all sheltered under a crude stone funerary structure. At some point during the past 5,000 years, it collapsed, crunching the skeletons into a solid layer of ivory-colored carnage. Skulls popped off their vertebrae. Limbs went askew. Mixed in with them were arrowheads, blades, and polished stone axes.

When Fernández-Crespo and her team examined the craniums, they found 107 injuries, primarily blows inflicted on the top of the head; they also found 22 limb fractures. Male skeletons were much more likely to be marked with these telltale signs of trauma than those of women. “It’s a beautiful piece of evidence,” Elizabeth Arkush, an archaeologist of war at the University of Pittsburgh, who was not involved in the study, told me. “The scale of this violent encounter is really impressive.”

When these killings occurred, vineyards didn’t yet exist around Laguardia, but people in the region were already growing wheat and barley. Fernández-Crespo believes that groups of farmers, each a few hundred strong, fell into a long-running conflict whose victims filled up the mass grave. If so, they’d be following a tradition that was already millennia old. We know that wars among rival farming groups were happening at least 2,000 years before the massacre outside Laguardia, thanks to a trio of gruesome sites in Germany. All three involved peoples from the Linearbandkeramik (LBK) culture, who are thought to have been the first settled farmers in Central Europe. At an LBK site southwest of present-day Nuremberg, archaeologists found the skeletal remains of 34 people in what has come to be called the Talheim death pit. Nearly half of the remains had belonged to children, and almost all showed strikes atop the head, most likely from an adze, a thick hoe-like tool.

At another LBK site northeast of Frankfurt, more than 25 adults and children also appear to have been massacred. Graves from this culture usually contain only one body that has been ritually cared for, and in many cases laid to rest with burial goods. These bodies were slammed together, haphazardly, and many of their limbs bore evidence of torture or, at a minimum, post-mortem mutilation. At Herxheim, an even grislier site further south, bones belonging to an estimated 500 individuals were found in the middle of another large LBK village. Skulls were systematically split, and, according to one interpretation, limbs were cut specifically for marrow extraction before being tossed into an oval pit.

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Our archaeological record of the early age of agriculture is spotty, which is why it’s so difficult to generalize about the existence of violence during that period, much less its prevalence. It’s all the more so when we push back in time to the hunter-gatherers of 10,000 years ago. Not only have more millennia passed since people from these cultures walked the earth; nomadic peoples tend to leave behind a thinner material legacy. They built fewer forts and defensive sites. Their weapons were repurposed hunting tools, meaning none indicate violence just by virtue of their existence. “For non-settled peoples, we are really restricted to using skeletal remains,” Arkush told me.

A cluster of 12 skeletons dug up from a 10,000-year-old site in the desert scrublands at Nataruk, Kenya, represents the only surefire evidence of war that has come down to us from this period. The bodies they belonged to ended up in the shallows of a lagoon that once covered the area. Ten were killed violently; one skull still has an obsidian blade lodged inside. The other two skeletons—which belonged to a man and a woman in the final months of her pregnancy—show signs of the bodies having been bound. Another large set of skeletons was found at a roughly 13,000-year-old site in present-day Sudan. But the bones were buried at a cemetery of sorts, and although some appear to have been felled by arrows, archaeologists disagree about how many of them, if any, were victims of war.

No convincing mass-casualty site has come down to us from before 15,000 years ago, when the deep chill of the Pleistocene finally ended. During the Ice Age, modern humans evolved and spent hundreds of thousands of years spreading to every continent save for Antarctica—but they didn’t leave behind any pits of bashed-in skulls, or at least not any that we have found. Some commentators have noted the intriguing absence of warfare in these peoples’ cave art, which otherwise depicts a great deal of human-on-animal violence. (Rock art from more recent periods includes some scenes of humans shooting arrows at one another, and warfare was of course a fixture in the iconography of ancient Egyptians, and nearly every large civilization since.) Even so, the evidence base is small. Few cave paintings have come down to us from the Ice Age, and the scholars who study them do not agree on their cultural purpose.

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In the absence of material remains, other anthropologists have tried to reason by analogy. They’ve surveyed more recent hunter-gatherers—in places such as Australia, the High Arctic, and New Guinea—in the hope that their behavior might tell us something about our earliest nomadic human ancestors. This approach has its limits, and not only because these peoples’ cultures have evolved since the Ice Age. Sample sizes are small. Collecting data requires making contact with tribes in ways that can bias findings. To complicate matters further, anthropologists have found that a tribe’s tendency to make war might depend on whether its people live near other hunter-gatherers or an agricultural settlement.

Scientists who have sought out some essential warring nature in our primate lineage have also been frustrated by a mixed record. Chimpanzees conduct raids, during which a group will kill a weakened or isolated individual in a neighboring territory. But they mainly do it when they stand very little chance of being hurt themselves. Unlike in human warrior cultures, self-sacrifice among chimps doesn’t seem to be individually rewarded with access to status or sex. Even if we grant that chimpanzees are warlike, they aren’t our only extant relatives. Bonobos are approximately as close to us genetically, and they don’t appear to engage in raids or the same degree of coalitional violence as chimps.

If we can’t be sure whether wars were occurring among Ice Age humans or our rival hominids—the disappearance of the Neanderthals may itself be evidence of war—we likely have no prayer of finding out whether our more ancient progenitors fell into group conflicts. Remarkably few of our fellow animals engage in organized violence, but some do. Wolves make lethal, chimplike raids on other packs during territorial disputes. Ants mobilize enormous armies to invade rival colonies, and they even take slaves. But these creatures sit on more distant branches of life’s tree than even our fellow primates. Their behavior can’t tell us whether war is hardwired into us.

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At a certain point, the evolutionary trail goes cold, and perhaps that’s for the best. There is danger in making too much of humanity’s immutable nature, and folly in emphasizing only our worst aspects. What separates us most from other species is our cultural plasticity: We are always changing, sometimes even for the better. We have largely given up chattel slavery. We have found ways to end blood feuds that implicated hundreds of millions. War may be a long-standing mainstay of human life, an inheritance from our deepest past. But each generation gets to decide whether to keep passing it down.